Post-mold cooling of molded articles is well known and used because thick walls of molded articles, such as preforms, retain heat from the injection molding process. Preforms are usually molded from polyethylene terephthalate (PET) resin and have wall thicknesses in the order of 2.0 mm or more. Because of the resin's very poor thermal conductivity, a substantial amount of residual heat is retained within the preform wall after it has been ejected from the mold. This heat migrates to the inner and outer surfaces of the preform and if not removed, while the preform is held in a form, would cause the preform's surfaces to reheat to the extent that its shape would alter significantly. Further, should the preform be touching another preform, this heat can cause them to weld together.
U.S. Pat. No. 4,592,719 teaches a post mold cooling device comprising a tube that is inserted into a freshly molded preform. The tube extends to the closed end of the preform such that when air is drawn through the tube ambient air is drawn into the interior of the preform from its open end causing an annular flow within the preform, the air reaches the closed end of the tube and continues to flow within the tube to be exhausted via a conduit provided within the tubes mounting plate. Thus the ambient air flow removes heat from the internal surface of the preform via an annular flow stream.
U.S. Pat. No. 4,729,732 teaches a post mold cooling tube into which a freshly molded preform is inserted to continue cooling. A vacuum source is provided at the closed end of the tube to cause the preform to slide toward the closed end as its outer diameter shrinks due to the cooling. The internal surface of the cooling tube is tapered to match the draft angle of the molded preform, so as the preform slides further into the tube its outer surface continues to maintain contact with the inner surface of the tube and continues to transfer heat to the cooling tube. This design was dubbed an “intimate fit” cooling tube, and is widely used today.
U.S. Pat. No. 6,475,422 B1 teaches a cooling pin inserted into a preform while it is being cooled in an intimate fit cooling tube. The pin is a hollow tube that extends near to the closed end of the preform and directs a cooling fluid (air) against the preform's inner surface at the closed end. The fluid then forms an annular cooling stream as it moves from the closed end of the preform toward its open end and vents to atmosphere. This stream of annular flowing air removes heat from the preform's inner surface.
JP 7-171888 teaches cooling the interior of a preform while it is being cooled in a cooling tube by blowing a jet of cooling air from a nozzle spaced apart from the open end of the preform. The nozzle directing the jet of cooling air is aligned coaxially with the longitudinal axis of the preform and the jet of cooling air travels parallel to this axis toward the closed end of the preform. This document also teaches alternately locating the nozzle so as to direct the jet of cooling air parallel to, and along, the inner surface of the preform.
U.S. Pat. No. 6,802,705 B2 teaches cooling the external neck finish of a preform while it is being cooled in an intimate fit cooling tube. A nozzle located near the preform's open end is angled to direct a jet of cooling air such that it flows around the thread finish formed on the external surface of the preform's neck portion to cool that surface. The stream of cooling air follows the spiral shape of the thread finish flowing between, and over, the crests of the thread. There is no teaching of cooling the preform's inner surface by this means.
All these prior art examples of preform cooling, by means of directing a jet of cooling air coaxially along the preform's surface, illustrate laminar flow convection cooling.